30% is the maximum. That is close. It is not a 'rare' as in the sense of this forum, but one does not see it very often!
I believe the only reason for the exceptional mod would be making spellbooks, as nothing else of note can be made exceptional with the inscription skill. (Runebooks, sure, but nobody cares.)
However, as far as I know, there is no benefit to an exceptionally-marked spellbook.
The only benefit that is rumored to exist would be a hidden one, that folks wearing a 30/30 talisman craft better spellbooks (a larger percentage of them have 3 mods). This is akin to crafters swearing that high luck suits improve their odds -- unproven, and in fact, argued against by people who can read the server code. Is that the case with this talisman rumor? I have no idea.